1. Field of the Invention
The present invention relates to a reseat system as a component of a touch signal probe installed in a coordinates measuring machine. More specifically, it relates to an arrangement of a reseat system for improving reproducibility of the return position of the reseat system during the return movement after the escape movement of the reseat system.
2. Description of Related Art
In a coordinates measuring machine, a probe movable in three-dimensional directions touches a workpiece on a fixed table and coordinate values of the respective axes (respective axes in the three-dimensional directions) when the probe touches the workpiece are read so that the dimensions and configuration of the workpiece are measured based on the coordinate values. Accordingly, a touch signal probe capable of indicating contact between a probe and a workpiece as an electric touch signal is widely used.
FIG. 6 shows a conventional touch signal probe. In the figure a stylus 1 is fixed to a movable component 2. A contact ball 4 is provided at a distal end of the stylus 1. Three cylindrical bodies 3 radially project at 120 degree intervals around an axis of the stylus 1 from the periphery of the movable component 2 on a plane perpendicular to the axis of the stylus 1. On the other hand, a fixed component 5 has three pairs of V-shaped holding portions 6 positioned corresponding to the cylindrical bodies 3 of the movable component 2. The cylindrical body 3 and the V-shaped holding portions 6 constitute a reseat component for defining the relative position of the fixed component 5 and the movable component 2 at one place.
According to the above arrangement, the movable component 2 is pressed to the fixed component 5 by virtue of a biasing force F of a biasing component (not shown) and the movable component 2 is forcibly brought into contact with the fixed component 5 through the reseat component. When pressing force from the workpiece is not applied to the distal end of the stylus 1, the movable component 2 rests on the fixed component at six contact points. In other words, respective cylindrical bodies 3 of the movable component 2 each rest on the respective cylindrical bodies 6 at two points for a total of six points. Accordingly, the reseat system is called as a six-point contact reseat system.
According to the six-point contact reseat system, the reseat position of the movable component after an escape movement can be located at only one place. In other words, assuming that the stylus 1 is displaced parallel to the axial direction at the rest position of the stylus 1 while maintaining contact between the reseat component on the movable component side and the reseat component on the fixed component side toward respective contact points, respective loci drawn by the tip of the stylus cross the axis of the stylus at the rest position. According to this arrangement, the stylus 1 returns to a unique rest position by restoring contact with the respective contact points by the biasing force F from a biasing component (not shown). When a pressing force from a workpiece is not applied to the distal end of the stylus 1, the movable component 2 rests on the fixed component 5 at six contact points. In other words, respective cylindrical bodies 3 of the movable component 2 rest on the V-shaped holding portions 6 at two points for a total six points. Accordingly, such arrangement is called a xe2x80x9csix-point contact reseat system xe2x80x9d. The six-point contact reseat system can use a pair of hard balls 7 as reseat component on the fixed component side (FIG. 7). Alternatively, one hard ball 8 may be used as a reseat component on the movable component side with a V-shape groove 9 provided on a surface of the fixed component as the reseat component on the fixed component side (FIG. 8).
According to the six-point contact reseat system, the return position after an escape movement of the fixed component can be defined at one place. Specifically, assuming that the stylus 1 displaces parallel to an axial direction of the stylus 1 at rest position toward respective contact points while keeping contact between the reseat component on the movable component side and the reseat component on the fixed component side, respective loci drawn by the tip of the stylus crosses the axis of the stylus at the rest position. According to the above arrangement, the stylus 1 returns to a uniquely defined rest position only by restoring contact between the respective contact points by the biasing force F during return movement of the movable component 2 after the escape movement by virtue of the pressing force from the workpiece, so that the rest position of the stylus 1 can be maintained at one place.
Since the position of the movable component relative to the fixed component can be set unique by the six-point contact reseat system., the six-point contact reseat system has high anti-vibration rigidity. Further, irrespective of the direction of the outside pressing force, the six-point contact reseat system has high reseat ability in a relatively rough unit of, for instance, 10 xcexcm.
However, the above-described six-point contact reseat system causes an error (xe2x80x9creseat shift errorxe2x80x9d) in a further fine unit of, for instance, 1 xcexcm observed in the return movement after contact, the error being caused because the movable component is pushed by the workpiece during the escape movement of the movable component to cause displacement relative to the fixed component.
Specifically, as shown in FIG. 9(A), when the contact ball 4 of the conventional reseat system touches the workpiece W, the stylus 1 moves in left direction in the figure as shown in FIG. 9(B). At this time, a small reaction force is caused between the movable component 2 and the fixed component 5, so that the movable component 2 slightly slides in the left direction in the figure. When the workpiece W and the stylus 1 connected to the movable component 2 are no more in contact with each other as shown in FIG. 9(C), the movable component 2 conducts the return movement by virtue of the biasing force F, where the axial position of the movable component 2 is shifted on account of the aforesaid slide movement. The shift directly affects on measurement accuracy of the probe.
The Applicants of the present invention have proposed a reseat system capable of correcting the reseat position shift after return movement (European Patent Publication No. 0764827 A2), whereby an electric voltage is applied to a piezoelectric element provided on the fixed component to administer directions of frictional force applied to a reseat system.
However, according to the above arrangement, the reproducibility of final return position is improved only by actuating the piezoelectric element after causing the reseat shift. Therefore, a measurement wait time is necessary before correction, thus hindering the efficiency of measurement work.
Further, since parts costs are increased for the piezoelectric element and an alignment process is necessary for attaching the piezoelectric element, the total production cost increases.
An object of the present invention is to provide a reseat system capable of restraining reseat shift upon returning from an escape movement to obtain an accurate return to the reseat position without adding a separate mechanism.
A reseat system according to the present invention includes: a fixed component; a movable component having a stylus to be settled at a predetermined rest position; first reseat components provided at three places on the fixed component mutually spaced apart; and second reseat components provided on the movable component to be in contact with the first reseat components at predetermined contact points, where, assuming that a first reseat component and a second reseat component corresponding thereto keep contact with each other and the stylus moves while keeping an axis of the stylus substantially parallel with an axis thereof at the rest position, a locus drawn by the tip of the stylus crosses the axis of the stylus at the rest position; and where, when an outside force is applied to the movable component, the tip of the stylus of the movable component can be displaced relative to the fixed component by releasing contact of either one of a first and second reseat component and, when the outside force is ceased to be applied to the movable component, the movable component returns to the rest position by a biasing force applied to the movable component in a direction from a base end of the stylus to a distal end thereof; the reseat system being characterized in that the locus drawn by the tip of the stylus is inclined in the direction of the biasing force moving away from the-axis of the stylus at the rest position.
According to the above arrangement, the reseat shift in returning from the escape movement can be effectively restrained.
Specifically, in the conventional reseat system, assuming that the stylus displaces parallel to the axis of the stylus at the rest position while keeping contact between the reseat system on the movable component side and the reseat system on the fixed component side, the locus drawn by the tip of the stylus extends in a direction perpendicular to the axis of the stylus at the rest position. Accordingly, little reaction force reverse to a component of the pressing force applied to the contact point is generated, thus causing the reseat shift in the escape movement.
On the other hand, according to the reseat system of the present invention, assuming that the stylus displaces parallel to the axis of the stylus at the rest position while keeping contact between the reseat system on the movable component side and the reseat system on the fixed component side, the locus drawn by the tip of the stylus is inclined in the direction of the biasing force moving away from the axis of the stylus at the rest position, so that the reaction force reverse to a component of the pressing force applied to the contact point is generated at the contact point and the reseat shift in the escape movement can be restrained.
In the present invention, when an angle between the locus (L2) drawn by the tip of the stylus and a plane perpendicular to the axis of the stylus at the rest position is represented by xcex8, xcex8 may preferably satisfy the following inequality of:
(Fp*sin(xcex1+"PHgr")sin(xcex1xe2x88x92xcex8)+Ft*cos(xcex2xe2x88x92"PHgr")sin(xcex2+xcex8))*xcexc greater than |Ft*cos(xcex2xe2x88x92"PHgr")cos(xcex2+xcex8)xe2x88x92Fp*sin(xcex1+"PHgr")cos(xcex1xe2x88x92xcex8)|
where xcex1 represents an angle formed by a straight line connecting a point on which the biasing force is applied to the movable component and a contact point of the first and the second reseat component and the plane perpendicular to the axis of the stylus at the rest position;
xcex2 represents an angle formed by a point on which the outside force is applied to the stylus and the contact point of the first and the second reseat component and the plane perpendicular to the axis of the stylus at the rest position;
"PHgr" represents an angle formed by the axis of the stylus when the outside force is applied to the stylus and the axis of the stylus at the rest position thereof;
Fp represents a magnitude of the biasing force;
Ft represents a magnitude of the outside force; and
xcexc represents friction coefficient of the first and the second reseat component.
Since the inclination angle of the locus drawn by the tip of the stylus satisfies the above relationship, an object of the present invention, i.e., prevention of reseat shift in return movement of the movable component can be effectively achieved.
Specifically, when the stylus relatively moves and touches the workpiece, the movable component is pushed in the outside force direction by the angle "PHgr" by receiving the outside force from the workpiece. At this time, biasing force Fp works on the application point of the biasing force and the pressing force Ft as the outside force works on the contact point of the stylus and the workpiece respectively. The forces are transmitted to the contact point of the reseat component on the fixed component side and the reseat component on the movable component side through the stylus. At this time, a component of the biasing force applied to the contact point of the reseat component on the fixed component side can be represented as Fp*sin(xcex1+"PHgr") and a component of the outside force can be represented as Ft*cos(xcex2+"PHgr").
Therefore, the pressing force applied on contact point of the reseat component on the fixed component side and the reseat component on the movable component side can be obtained by resolving and compositing the force applied to the contact point of the reseat component on the fixed component side and the reseat component on the movable component side in perpendicular direction relative to a plane for the movable component to move along when the tip of the stylus moves along the locus (referred to contact surface hereinafter). Here, friction force on the contact point of the reseat component on the fixed component side and the reseat component on the movable component side can be represented by multiplying the pressing force and friction coefficient xcexc, specifically as follows:
Friction force applied on the contact point=(Fp*sin(xcex1+"PHgr")sin(xcex1xe2x88x92xcex8)+Ft*cos(xcex2xe2x88x92"PHgr")sin(xcex2+xcex8))*xcexc
Further, a force for causing the reseat shift of the movable component can be obtained by resolving and compositing the force applied to the contact surface parallel to the contact surface. The component of force parallel to the contact point is given as an absolute value of difference between a force as a component of the outside force for sliding in outside force application direction and a force as a component of the biasing force for sliding to the outside of the reseat system, which is specifically represented as follows:
Component of force parallel to the contact surface=force for causing reseat shift=|Ft*cos(xcex2xe2x88x92"PHgr")cos(xcex2+xcex8)xe2x88x92Fp*sin(xcex1+"PHgr")cos(xcex1xe2x88x92xcex8)|
In the above, when a is small (the application point of the biasing force is adjacent to the workpiece) or "PHgr" is small (the movable component is pushed by the outside force only for small amount at initial stage of contact), the force for causing reseat shift in the application direction of the outside force is dominant as described above. Accordingly, xcex8 has to be defined for a certain magnitude.
On the other hand, when xcex1 is large (the application point of the biasing force is remote from the workpiece) or "PHgr" is set large (the movable component is greatly pushed after contact), a force for sliding the movable component to the outside (in a direction approaching to the workpiece) of the reseat system by virtue of the biasing force is dominant. Accordingly, xcex8 has to be defined within a predetermined range.
Therefore, when the predetermined xcex1 and xcex2 are given according to a construction of the reseat system, and the biasing force Fp, the outside force Ft applied to the stylus and maximum push angle "PHgr" are estimated according to a measurement condition, the relationship of (friction force applied on the contact point) greater than (force for causing reseat shift) can be satisfied by setting the inclination angle xcex8 in the above-described manner. Therefore, the reseat shift during contact and return movement can be effectively prevented.
In the above arrangement, either one of the first and the second reseat component may preferably be a cylinder having larger outer diameter on a side remote from the stylus than outer diameter on a side adjacent to the stylus and the other may preferably be a pair of balls.
Accordingly, assuming that the stylus displaces parallel to the axis of the stylus at the rest position while keeping contact between the reseat system on the movable component side and the reseat system on the fixed component side, the locus drawn by the tip of the stylus is inclined in the biasing force direction as being separated from the axis of the stylus at the rest position in accordance with the inclination of the circumference of the cylinder having larger outer diameter on the side remote from the stylus than the outer diameter on the stylus side (so-called tapered surface). Therefore, the present invention can be effectively implemented.
Alternatively, either one of the first and the second reseat components may preferably be a cylinder having predetermined inclination in an axial direction and the other may preferably be a pair of balls.
Accordingly, assuming that the stylus displaces parallel to the axis of the stylus at the rest position while keeping contact between the reseat system on the movable component side and the reseat system on the fixed component side, the locus drawn by the tip of the stylus is inclined in the biasing force direction as being separated from the axis of the stylus at the rest position in accordance with inclination of the cylinder having predetermined inclination in the axial direction. Therefore, the present invention can be effectively performed only by partially changing the construction of the reseat system while commonly using the parts and production process of the conventional reseat system.
In the present invention, the other of the first and the second reseat components may preferably be a pair of cylinders arranged in a V-shape. With the pair of cylinders, the six-point reseat construction can also be secured, thus effecting the present invention.
Further, in the present invention, either one of the first and the second reseat components may preferably be a ball and the other may preferably be a V-shaped groove having a predetermined inclination in an axial direction.
Accordingly, assuming that the stylus displaces parallel to the axis of the stylus at the rest position while keeping contact between the reseat system on the movable component side and the reseat system on the fixed component side, the locus drawn by the tip of the stylus is inclined in the biasing force direction moving away from the axis of the stylus at the rest position in accordance with inclination of the V-shape groove having predetermined inclination in the axial direction. Therefore, since the reaction force reverse to the component of the pressing force applied on the contact point is generated on the contact point, the reseat shift in returning from the escape movement can be effectively restrained.